The invention relates to a magneto-optical light switching element comprising islands which are formed from an epitaxial layer on the basis of bismuth-substituted rare earth metal iron garnet, which layer is provided on a magnetically unordered and optically transparent garnet substrate, integrated heating resistors being provided on the islands which are surrounded by a coil.
The invention further relates to a method of manufacturing such a magneto-optical light switching element and to its use.
For processing texts, graphs and pictures ever more use is made of electronic data processing. To output the information, fast, high-resolution printers such as, for example, electrophotographic printers having optical printing heads are used.
In Technische Information 84 07 16 published by Valvo, a magneto-optical light switching module is described which can suitably be used for the formation of such compact, optical, high-resolution printing heads.
The known modules contain a series of punctiform light switching elements which can independently be switched between a transparent and an opaque condition in a thermo-magnetic way.
In the manufacture of the light switching elements a (111)-oriented monocrystalline substrate of substituted gadolinium-gallium-garnet is employed. A layer of bismuth-substituted rare earth metal-iron-garnet of the qualitative composition (Gd,Bi).sub.3 (Fe,Ga,Al).sub.5 O.sub.12 is applied to this substrate by means of epitaxy. Such layers and substrates are known from J. Cryst. Growth 64 (1983), pages 275 to 284. In order to obtain single light switching elements, most of the magneto-optical layer initially covering the entire substrate is removed by means of a photolithographic mask-etching process, so that only single islands remain. Each island forms the basis of a light switching element. In a further masking process the interval between the light switching elements is covered with an opaque layer. Thus, only the light switching elements pass light. During the time that the substrate is not ordered magnetically and is optically inactive, the magneto-optical layer exhibits a spontaneous magnetisation which due to the non-statistic distribution of the bismuth irons in the crystal lattice is always oriented perpendicularly to the layer surface, i.e. the magnetisation can only have two directions: either parallel or antiparallel to the normal to the layer.
When linearly polarised light passes through this layer, the plane of polarisation of this light is rotated either clockwise or anti-clockwise, dependent upon the direction of magnetisation, due to the Faraday effect. This rotation of the plane of polarisation is converted to an intensity modulation by a polarisation-sensitive optical system. For this purpose the layer is arranged between two polarising foils, i.e. foils which only transmit light of a predetermined plane of polarisation. The first foil (polariser) is used to filter linearly polarised light from incident light, and the second foil (analyser) is used to block the light of one of the planes of polarisation. Light whose plane of polarisation corresponds to the other direction of magnetisation is transmitted by the light switching elements. Consequently, a reversal of the direction of magnetisation leads to a change-over from the opaque to the transparent condition or conversely.
So long as the dimensions of the light switching element do not exceed a certain critical value of approximately 500 .mu.m, a uniform direction of magnetisation is always formed within the light switching element; with light switching element dimensions in the range below 100 .mu.m, as they are used for printing head applications, the direction of magnetisation is very stable.
On the other hand, the directional stability of the magnetisation is dependent upon the temperature, at temperatures over 150.degree. C. the stability decreases drastically. This effect is used for switching. To this end, a resistor element is provided by means of the thin-film technology in one corner of each light switching element and is connected via a track network to an electronic drive circuitry. Moreover, a coil formed of a readily conceived wire turn is arranged so that it surrounds all light switching elements of a light switching array. A light switching element is switched by applying a current impulse of approximately 15 .mu.s to the resistor. Consequently, the temperature in the vicinity of this resistor element (heating element), rises to over 150.degree. C., thereby strongly reducing the magnetisation stability in the material below the resistor element. By means of the coil a magnetic field of approximately 20 kA/m is then produced for approximately 10 .mu.s. Under the influence of said field the magnetisation in the heated area is directed towards the external magnetic field. In this way a "nucleus" for a new magnetic domain is formed. This will grow under the influence of the magnetic field which remains activated for a few more microseconds, until it covers the entire light switching element, thereby again exhibiting a uniform magnetisation.
For the proper functioning of such a light switching element the amount of Faraday rotation, the compensation temperature and the lattice constant of the epitaxial layer, as well as the so-called uniaxial magnetic anisotropy K.sub.u of the layer material are of great importance. Experience has shown that epitaxy layers of bismuth-substituted yttrium or gadolinium iron garnet which are grown in (111)-direction, exhibit a substantial degree of positive, growth-induced anisotropy K.sub.u which in layers deposited from a melt increases as the bismuth content is higher. A positive growth-induced anisotropy K.sub.u ensures that the magnetisation vector is perpendicular to the layer surface, as is required for the proper functioning of the light switching element described above. On the other hand, in a thermomagnetic switching process the force which determines the preferred direction of magnetisation must be overcome by an external magnetic field. Due to the materials used so far for the production of known light switching elements, this so-called magnetic switching field is so large that it can only be provided by an external wire-wound coil which is provided around the magneto-optical islands by means of hybrid technology, and not by an integrated coil because its cross-section is too small, as a consequence of which the current density is raised.
As a separate production cycle is necessary for the manufacture of said coil, magneto-optical light switching elements are expensive and the electronic drive circuitry for producing the necessary high currents is relatively costly. A further disadvantage of the use of a separate wire-wound coil is that due to power dissipation in the coil wire the switching rate of the single light switching elements is limited to an array frequency of 2 kHz.